Method of producing endodontic instruments

ABSTRACT

Methods for manufacturing endodontic instruments are provided. Methods relate to milling a blank with a cutting process to shape the instrument. Instruments so produced are also provided.

The invention relates to a method for producing endodontic instrumentsfrom a blank in rod or wire form.

Endodontic instruments are used in particular in a dental root canaltreatment. They are used to treat the diseased root canal. Theseinstruments may, for example, be small reamers, drills and files of avery wide range of geometric shapes and designs.

Endodontic instruments are usually manufactured from blanks in rod orwire form with a diameter of approximately 0.3 to 3 mm. Theseinstruments are relatively complex to produce, requiring complexabrasive processes. It is attempted, by using grinding wheels which areadapted in a complicated way to the process and have specially definedparameters, to achieve satisfactory results.

It is known from U.S. Pat. No. 5,655,950 to manufacture endodonticinstruments from a metal alloy comprising nickel and titanium fractionsby grinding. Parameters such as the relatively low feed rate of, forexample, up to 250 mm/min, with the need to precisely maintain thecircumferential speed of the grinding wheel, lead to a highmanufacturing outlay for each instrument. A drawback of this method isthat microcracks are easily formed on account of the superelasticity ofthe nickel-titanium alloys. The formation of microcracks is alsopromoted by the fact that the abrasive grains do not have a regulargeometry and a high grinding speed is used.

The machining of metal blanks with dimensions of larger than a fewmillimeters by metal-removing shaping with a geometrically definedcutting edge and cutting removal, for example by milling, drilling,cutting or turning, is known in other engineering fields. However, inthis case, the success of metal-removing machining of metal blanks bymilling is dependent on the materials properties of the metal blank.

It is also known to machine metal blanks by high-speed-cutting milling(HSC milling). This milling method is distinguished by a high cuttingspeed, which allows heat to be dissipated via the chips which areremoved.

The invention is based on the object of producing high-qualityendodontic instruments in an economic way.

According to the invention, this object is achieved by the provision ofa method as described in the claims.

The endodontic instruments are in this case produced by milling methodsusing milling cutters with a geometrically defined cutting edge andcutting removal, in particular by high-speed-cutting milling.

The method according to the invention has the following advantages:

While in the known manufacture of endodontic instruments by grinding theknown grinding wheels become clogged by shaving removal of material, inthe method according to the invention the cutting removal of materialmeans that the cutting edges of the milling cutters do not becomeblocked with chips. This allows the precise and defined production andmachining of a large number of endodontic instruments without themilling cutters having to be cleaned and/or replaced.

For example, when using titanium-coated hard metal milling cutters, itis possible to achieve a particularly high service life while thecutting quality remains constant. This minimizes tool costs during themanufacturing process.

A further advantage is that the microcracks which are often observedduring grinding in the machined material can be substantially avoidedwhen using the present method.

The wire or blank is preferably guided in a special holder and can beheld in an optimum position with respect to the cutting edges of themilling cutter by means of a support device. This holder preferablycomprises a perforated disc for defined guidance of the wire. In thiscase, the milling cutter is preferably fitted slightly eccentrically, inorder to prevent poor-quality removal of material at the lowcutting-edge speed in the center of the milling cutter. The supportdevice preferably has a block with a channel which is matched to thediameter of the blank or workpiece. The blank is preferably guided andsupported in this channel during the machining, so that it cannot escapefrom the milling cutter. The block can be tracked in the radialdirection by means of a spring, by means of a servomotor, hydraulicallyor pneumatically, in order to compensate for any reduction in diameterwhich could result from the milling. The tracking of the support deviceexpediently takes place elastically and radially with respect to theblank in rod or wire form.

Even at high cutting speeds in the range from 10 to 300 m/min,preferably in the range from 50 to 200 m/min, with the associated rapidfeed rates, the amount of heat introduced into the material is low,since the milling method means that the heat which is generated isdissipated via the chip. Consequently, the changes in the microstructureof the material, despite high and economical removal rates, are very lowand therefore do not cause any deterioration in the original materialsproperties. The superelasticity of the endodontic instrument, which isdesired by the user, is fully retained.

Furthermore, the surface quality in the micrometer range is of very highquality, since the formation of notches in the material is greatlyreduced by the cutting machining. Consequently, it is possible togenerate the exact cutting edges of defined sharpness without causingspalling, resulting in a positive quality in terms of sharpness andcutting capacity for the user.

Finally, the method according to the invention makes it possible toachieve unexpectedly high feed rates of the workpiece of up to 2000mm/min, so that the production time for each instrument can be reducedconsiderably. Feed rates in the range from 20 mm/min to 1000 mm/min haveproven advantageous.

In addition, it is possible to complete the entire removal of materialdown to the desired depth in order to form the shank or the fluteswithin a single feed operation, so that it is not necessary, as is oftenthe case with grinding methods, to pass over the same part of the blankor wire two or three times. For these reasons, the number of instrumentswhich it is possible to produce each year per machine is higher by afactor of up to five than with conventional grinding methods, eventhough the machine investment costs are similar.

When using CNC milling machines (CNC=computer numerical control), it ispossible to achieve any conceivable form of endodontic instruments witha single type of machine by simply changing the program. Simplyadjusting the manufacturing parameters (changing the program) eliminatesthe need for expensive, machine-specific refits.

HSC milling can be used in particular also for rapid and precisemachining of superelastic nickel-titanium alloys with high cuttingspeeds, which was unexpected. However, the method is also suitable formachining materials which have a high tensile strength and may also havebeen hardened, such as steel and graphite.

Further advantages of this method are the ease of producing even complexgeometries in a rod-shaped blank or wire by flexible positioning of themilling cutter. The method according to the invention can be used tomill all useful surface geometries, such as flutes, shanks and spirals.

Suitable milling tools are designed, for example, as two-tip end millswith standard cutting edges and are preferably coated with, for example,titanium nitrite (TIN), titanium carbonitride (TiCN), titanium aluminumnitride (TiAlN), CBN and polycrystalline diamond (PCD). A standarddiameter for a suitable face-milling cutter lies in the range from 0.3to 3 mm, preferably in the range from 0.5 to 2 mm. A setting angle forthe milling tools in the range from 5 to 30°, preferably in the rangefrom 10 to 20° C., has proven expedient.

The cross section of the blank in rod or wire form may be rounded(circular or elliptical) or polygonal (square, rectangular ortriangular).

The invention is explained below using exemplary embodiments, withoutthe intention being for the invention to be restricted by theseembodiments. In the drawing:

FIG. 1 shows a side view of an HSC milling machine, in which a blank isbeing machined by means of a centrally arranged milling cutter;

FIG. 2 shows a plan view of the HSC milling machine from FIG. 1;

FIG. 3 shows a side view of an HSC milling machine, in which a blank isbeing machined by means of an eccentrically arranged milling cutter;

FIG. 4 shows a plan view of the HSC milling machine shown in FIG. 3;

FIG. 5 shows a detailed view of FIG. 4 encompassing the milling regionincluding milling cutter and support device;

FIG. 6 shows a side view of an HSC milling machine, in which a blank isbeing machined by means of a milling cutter which is set at an angle;

FIG. 7 shows a plan view of the HSC milling machine shown in FIG. 6;

FIG. 8 shows a side view of an HSC milling machine, in which a blank isbeing machined by means of a milling cutter which is set as acylindrical milling cutter;

FIG. 9 shows a plan view of the HSC milling machine shown in FIG. 8.

FIG. 10 shows a support device with a spring to support the blank.

In the exemplary embodiments illustrated in FIGS. 1 to 9, a blank 7which is in rod form and consists of a superelastic nickel-titaniumalloy is being machined in a milling machine by means of an HSC millingcutter 4. The milling machine has a base plate 6, on which a guidereceptacle 3 with an integral cutting device 8 and with a continuoussupport device 5 for guiding the blank 7, and a conventional feed unitcomprising chuck 1 and outer support 2 are accommodated. The millingcutter 4 is held by a conventional milling cutter spindle, which is notshown. Its working area is precisely in the region of the support device5.

During the machining, the blank 7, which is illustrated on the left, ispulled onward over an entire machining length between the chuck 1 andthe outer support 2. The outer support 2 opens, and the rotatablymounted chuck 1 then pushes the blank 7 into the working area of themilling cutter 4 which provides the end of the rotating blank 7 with thedesired geometry. Then, the desired surface geometry is imparted to theblank 7. In the process, the milling path is generated by a rotarymovement of the chuck 1 in combination with a linear feed movement. Toensure a high level of precision, the machining takes place in a singlepass. When the machining has been concluded, the workpiece can be cutoff using the milling cutter 4 or a cutting device 8.

Different milling cutter geometries and positions relative to theworkpiece or different setting angles of the milling cutter 4 enabledifferent geometries to be formed in the blank 7. The variations allowthe cutting geometry of the endodontic instrument which is to bemanufactured to be optimized.

In the exemplary embodiment shown in FIGS. 1 and 2, the milling cutteris set at right angles to and centrally on the blank 7. The cuttingspeed is very low in the center of the milling cutter 4, so thatmicrocracks may form in the material of the blank 7 at this location.Such microcracks may be tolerable in some instruments.

The area of such microcracks can also be remachined in a second pass.

In the exemplary embodiment shown in FIGS. 3 to 5, the milling cutter isset at right angles but eccentrically. The area of the milling cutter 4which is close to the center therefore does not touch the blank, so thatthe cutting edges of the milling cutter 4 always have a high cuttingspeed at the points where they are in contact with the blank 7.

In FIGS. 6 and 7, the milling cutter 4 is set centrally but at an angle.

The milling cutter positions shown in FIGS. 1 to 7 are primarilysuitable for forming flutes. A cylindrical milling cutter 4 is used inFIGS. 8 and 9. This arrangement allows material to be removed inparticular from the circumference. This is also possible in thearrangements shown in FIGS. 1 to 5, by using an end milling cutter.

In the exemplary embodiment shown in FIG. 10, the support device 5 isprovided with a spring 5 a to elastically and radially support theblank.

The following parameters have proven practical for the machining:depending on the size of the milling cutter, the cutting speed V_(C) canbe selected between 20 and 200 m/min, the feed f_(Z) per tooth orcutting edge of the milling cutter is expediently between 0.5 and 100μm/tooth (combined movement comprising linear feed and rotation of theblank), the preferred values being V_(C) roughly≈50 m/min and f_(Z)roughly≈5 μm.

1. A method of making an endodontic instrument from a rod form blank ofa nickel-titanium alloy, comprising: milling said rod form blank using ahigh speed cutting process and a geometrically defined cutting edge of amilling cutter to shape said instrument by cutting material therefrom.2. A method according to claim 1, wherein said milling is performed at acutting speed in the range of 10 m/min to 300 m/min.
 3. A methodaccording to claim 1, wherein said milling is performed with the millingcutter fitted eccentrically above a longitudinal axis of the blank.
 4. Amethod according to claim 2, wherein said milling is performed with themilling cutter fitted eccentrically above a longitudinal axis of theblank.
 5. A method according to claim 1, comprising supporting the blankwith a lateral guide at a side opposite the milling cutter during saidmilling.
 6. A method according to claim 2, comprising supporting theblank with a lateral guide at a side opposite the milling cutter duringsaid milling.
 7. A method according to claim 3, comprising supportingthe blank with a lateral guide at a side opposite the milling cutterduring said milling.
 8. A method according to claim 4, comprisingsupporting the blank with a lateral guide at a side opposite the millingcutter during said milling.
 9. A method according to claim 1, whereinthe lateral guide elastically supports the blank during said milling.10. A method for machining and/or manufacturing endodontic instrumentsfrom a blank in rod or wire form, in which a milling method using ageometrically defined cutting edge and cutting removal is used forshaping.
 11. The method as claimed in claim 10, in which ahigh-speed-cutting milling method is used.
 12. The method as claimed inclaim 10, in which the cutting speed V_(C) is in the range from 10 m/minto 300 m/min.
 13. The method as claimed in claim 10, in which the blankis configured from a nickel-titanium alloy.
 14. The method as claimed inclaim 10, in which a milling cutter is fitted eccentrically above theworkpiece longitudinal axis.
 15. The method as claimed in claim 10, inwhich the blank is supported on the side which lies opposite a millingcutter.
 16. The method as claimed in claim 15, in which the blank issupported elastically.
 17. The method as claimed in claim 15, in whichthe support for the blank comprises a lateral guide.
 18. A methodaccording to claim 4, wherein the lateral guide elastically supports theblank during said milling.
 19. An endodontic instrument made by themethod of claim 1.